In the gas turbine industry, a common problem with structural turbine casings is distortion of the casing, e.g., out-of-roundness, caused by the response of the casing to various temperature and pressure conditions during turbine operation. Gas turbines undergo rapid thermal transient loading during normal operation that produce large thermal gradients in the casing structures. If the thermal mass distribution is non-homogeneous around the casing, then there will be a resultant distortion from the intended circular shape.
Typical turbine and compressor housings are formed in upper and lower halves connected one to the other along a horizontal plane by vertical bolts extending through radially outwardly directed and enlarged flanges at the housing splitline. These split-wall casings with large flanges running down the split-line joint result in a thermal mass concentration that can result in casing distortion during a thermal transient event. One reason for the casing to distort is that the mass of the splitline flange is large, causing it to respond thermally at a rate slower than the response time for the balance of the turbine housing. Coupled with this large mass is a large thermal gradient through the flange which causes the flange to pinch inwards due to thermally induced axial strain.
Distortion is a large component of setting stage 1 and 2 turbine clearances, which can be the most sensitive in the machine, and generally affect efficiency and output to the largest degree. Current gas turbines have large distortions during transient operations, which are generally the worst on a hot restart, and clearances are generally opened on a one-for-one basis to account for distortion, directly impacting steady state clearances. This type of distortion is an important component to setting steady state clearances for the stage 1 turbine rotor, and tighter clearances result in improved gas turbine operability and performance.
Additional distortion can result from the hoop load discontinuity at a split-line of a multi-piece casing. The total resulting distortion from the ideal circular shape is one factor in determining the minimum clearance between rotating and stationary parts, as the rotating parts can not expand beyond the minimum radius of the casing, even if this minimum radius exists over a very small portion of the casing. In order to provide for tighter clearances, the casing should be as circular as possible whenever the clearances are small. Minimum tip clearance results in less leakage of working fluid over the tip of the blade/vane which yields the highest efficiency operation of the gas turbine.
Another cause of distortion is a result of internal casing pressure. Further, it will be appreciated that there is an offset between the centerline of the bolt holes and the main portion of the turbine casing at the split-line flanges. Because of this offset, a moment is introduced by the hoop field stress transferred through the bolts, causing the split-lines to deflect radially inwardly.
To mitigate distortion, sometimes “false” flanges are used to provide additional thermal mass at other circumferential locations on the casing. U.S. Pat. No. 5,605,438 (“the '438 patent”) discloses casings for rotating machinery, such as turbines and compressors, which significantly reduces distortion and out-of-roundness through the use of “false” flanges. The '438 patent discloses a turbine casing that is provided with a strategically located circumferential rib and a plurality of axially extending flanges. The '438 patent also discloses a compressor casing that is provided with only a plurality of axially extending flanges. The entire contents of the '438 patent are incorporated herein by reference.
FIG. 1, which corresponds to FIG. 3 of the '438 patent, illustrates a generally semi-cylindrical turbine casing half 40 that mates with a similar semi-cylindrical casing half (not shown) at horizontal split-line flanges 42 by bolts in radially split bolt holes (not shown). To reduce the distortion of the turbine casing caused by internal pressure and to control the thermal response of the turbine during start-up and shut-down, each of the mating casing halves 40 is provided with a circumferentially extending rib 44. Rib 44 extends about each half of the cylindrical turbine casing between opposite ends thereof, terminating at its ends just short of the split-line flanges 42. By locating the rib 44 circumferentially about the semi-cylindrical halves, the distortion of the casing half caused by internal pressure is significantly reduced. Additionally, one or more axially extending flanges 46 are provided in each of the semi-cylindrical casing halves 40. As illustrated in FIG. 1, the casing half 40 is provided with three axially extending ribs 46 that are spaced circumferentially one from the other around casing half 40. These ribs 46 substantially match the stiffness and much of the thermal mass of the horizontal split-line flange 42. Because flange 42 has slots which run from the bolt hole to the outside surface of the flange, there is a reduction in strain in flange 42 which enables the axially extending ribs 46 to be designed smaller than the horizontal flange 42, i.e., the axial ribs 46 are not as massive as the split-line flanges 42. Because the split-line flanges 42 have the slots, the stiffness is reduced in a radial direction. The '438 patent teaches that only the radial stiffness of the split-line flanges 42 needs to be matched.
FIG. 2, which corresponds to FIG. 4 of the '438 patent, illustrates one half of a compressor casing in the form of a semi-cylindrical half 50 that mates with a similar semi-cylindrical compressor casing half (not shown) at horizontal split-line flanges 54. The compressor casing half 50 does not include a circumferentially extending rib because of a lack of significant thermally induced stresses in the compressor casing. However, one or more axially extending flanges are provided at circumferentially spaced positions about the housing half, similar to the turbine casing half 40 discussed above. The same considerations with regard to stiffness and the reduction in the size or mass of the axially extending flanges 52, as discussed above with respect to the axial flanges of the turbine casing 40, are applicable.
“False” flanges, similar to flanges 46 and 52 shown in FIGS. 1 and 2, have been used extensively, but they do not solve all distortion problems. They only address the thermal mass effect. The hoop stiffness under each of the “false flanges” does not match that at the split line due to the bolted joint stiffness discontinuity at, say, split-line flanges 42 shown in FIG. 1. It should be noted that the number of false flanges, such as flanges 46 and 52 shown in FIGS. 1 and 2, can be more than two in number.